Disc drives are used as primary data storage devices in modem computer systems and networks. A typical disc drive comprises one or more rigid magnetic storage discs which are journaled about a rotary hub of a spindle motor to form a disc stack. An array of read/write transducing heads are supported adjacent the disc stack by an actuator to transfer data between tracks of the discs and a host computer in which the disc drive is mounted.
Conventional actuators employ a voice coil motor to position the heads with respect to the disc surfaces. The heads are mounted via flexures at the ends of a plurality of arms which project radially outward from an actuator body. The actuator body pivots about a shaft mounted to the disc drive housing at a position closely adjacent the outer extreme of the discs. The pivot shaft is parallel with the axis of rotation of the spindle motor and the discs, so that the heads move in a plane parallel with the surfaces of the discs.
The actuator voice coil motor includes a coil mounted on the side of the actuator body opposite the head arms so as to be immersed in the magnetic field of a magnetic circuit comprising one or more permanent magnets and magnetically permeable pole pieces. When current is passed through the coil, an electromagnetic field is set up which interacts with the magnetic field of the magnetic circuit to cause the coil to move in accordance with the well-known Lorentz relationship. As the coil moves, the actuator body pivots about the pivot shaft and the heads move across the disc surfaces.
The control of the position of the heads is typically achieved with a closed loop servo system such as disclosed in U.S. Pat. No. 5,262,907 issued to Duffy et al. and assigned to the assignee of the present invention. A typical servo system utilizes servo information (written to the discs during the disc drive manufacturing process) to detect and control the position of the heads through the generation of a position error signal (PES) which is indicative of the position of the head with respect to a selected track. The PES is generated by the servo system by comparing the relative signal strengths of burst signals generated from precisely located magnetized servo fields in the servo information on the disc surface.
The servo system primarily operates in one of two selectable modes: seeking and track following. A seek operation entails moving a selected head from an initial track to a destination track on the associated disc surface through the initial acceleration and subsequent deceleration of the head away from the initial track and toward the destination track. A velocity control approach is used whereby the velocity of the head is repeatedly estimated (based on measured position) and compared to a velocity profile defining a desired velocity trajectory for the seek. Corrections to the amount of current applied to the coil during the seek are made in relation to the difference between the estimated velocity and the desired velocity.
At such time that the head reaches a predetermined distance away from the destination track (such as one track away), the servo system transitions to a settling mode wherein the head is settled onto the destination track. Thereafter, the servo system enters a track following mode of operation wherein the head is caused to follow the destination track until the next seek operation is performed.
Disc drive designs thus typically use proximate time optimal control with a velocity profile to control a selected head during a seek, a state estimator based controller with relatively slow integration to settle the head onto the destination track, and the same state estimator based controller with relatively fast integration for track following.
Typically, disc drive designers have employed ball bearing cartridges for journaling the actuator assembly about the pivot point. These bearing assemblies are subject to very rapid, repetitive movements of the actuator arm about the pivot point as the heads are radially moved from track to track. The precision of seeking and track following operations is dependent upon the performance of the actuator bearing assembly. As the storage capacity of modern disc drives continues to increase, the precision required by the rotation of the actuator arm about the bearing assembly also increases.
Despite the requirements for precise movement, ball bearing assemblies are subject to mechanical limitations that can adversely affect their use in today's high-performance disc drives. More specifically, conventional ball bearing assemblies are subject to metal wear, increased vibrational resonance and friction, and lubricant outgassing. Each of these limitations increases the presence of extraneous motion exhibited by the ball bearing assembly during rotation.
In concert with these mechanical limitations, ball bearing assemblies also provide an undesirable translational degree of freedom in the X-Y plane (i.e. a plane intersecting the assemblies and normal to the axes about which the assemblies rotate). This translation is caused primarily by the deflection of the ball bearings within the inner and outer races of the bearing assembly. The deflection of the ball bearings results from a lateral force applied to the actuator during a seek or track following operation. During deflection, the ball bearings exhibit a "spring-like" response to the laterally applied force. The natural frequency of the resulting bearing translation is dependent on the mass of the actuator arm and the spring stiffness of the bearing assembly. This vibration mode is often referred to as the bearing translation mode.
A variety of solutions have been proposed to limit the presence of translational modes of vibration in disc drive actuator bearings. For instance, adding mass to the actuator arm tends to reduce the frequency of the bearing translational mode. U.S. Pat. No. 4,812,935 issued to Sleger teaches the limitation of bearing translational modes through use of mass dampers. However, adding mass to the actuator arm has the unwanted side effect of slowing seek operations and limiting servo bandwidth. Other proposed solutions include absorbing the vibratory energy through use of elastomeric components within the bearing assembly, as taught by U.S. Pat. No. 5,983,485 issued to Misso and assigned to the assignee of the present invention. Although absorption components may reduce translational vibration, the additional space required for installing these components is prohibitive in modern, compact disc drives.
In light of the deficiencies presented by the prior art solutions, there continues to be a pressing need to develop a compact means for limiting the presence of bearing translation while improving the overall performance of actuator movement.